The present invention relates generally to procedures and equipment utilized in conjunction with subterranean well operations and, in an embodiment described herein, more particularly provides a method and associated system for setting downhole control pressure.
Various hydraulically controlled downhole tools are presently used in subterranean wells. A majority of these tools are flow control devices, such as valves and chokes, although other types of tools are also available which are hydraulically controlled. Pressure may be applied to the tools via one or more control lines which extend between the tool and a pressure source, such as a pump at the earth's surface.
Some of these well tools perform different functions or operate in different manners based on certain pressure levels applied to the tools, sequences of pressures at certain levels applied to the tools, or combinations of certain pressure levels in multiple lines connected to the tools, etc. In order to reliably operate the tools, an operator should accurately know what pressure is applied to the tool downhole. Note that an “application” of pressure can be an increase in pressure or a decrease in pressure as desired or as required by a particular control system.
Examples of hydraulically controlled well tools and methods of controlling operation of such tools are described in U.S. Pat. Nos. 6,470,970, 6,567,013 and 6,575,237. The disclosures of these prior patents are incorporated herein by this reference.
Unfortunately, in the typical case the control line is very long and has a relatively small flow area, and so there is significant resistance to transmission of pressure through the line. This means that pressure in the line measured at the surface is not necessarily the same as pressure in the line at the downhole well tool (even when corrected for hydrostatic pressure due to the fluid in the line). Instead, there is a significant time lag between application of a pressure to the line at the surface and a corresponding change in pressure in the line at the well tool.
Eventually, the pressure at the well tool will reach the pressure applied to the line at the surface (plus hydrostatic pressure in the line). However, it will take a very long time since the pressure at the well tool approaches the pressure applied to the line at the surface asymptotically.
Yet another complicating factor is that each well installation is different. The control line may be a different size or length, the fluid used in the line may be different, a temperature profile of the well may vary (which affects compressibility of the fluid in the line), air or other gases can be entrained in the fluid in the line, etc.
One solution to these problems is to install a pressure sensor at the well tool to directly measure the pressures applied to the tool. This does not solve the problem of the time lag between changing pressure at the surface and experiencing the changing pressure at the well tool, but at least the changed pressure can be measured at the well tool to determine whether a desired control pressure has been achieved.
Unfortunately, the use of a pressure sensor at the well tool brings with it another set of problems. For example, there is the expense and time required to install the pressure sensor. Provisions must be made for communicating with the sensor, such as via wireless telemetry, electrical or fiber optic lines, etc. The sensor and the communication system are subject to damage during installation and will likely need to be serviced periodically.
Therefore, it may be seen that a need exists for improved methods of setting downhole control pressures.